In the past, as a birefringence measuring device based on a phase shift method, a device introduced in a thesis “Current Situation of Digital Photoelasticity Method”, by Eisaku Umezaki, Experimental Dynamics Vol. 4, No. 1, March 2004, FIGS. 6 and 7 and described in “A Novel Instrument for Transient Photoelasticity” by J. W. Hobbs, R. J. Greene and E. A. Patterson, Experimental Mechanics Vol. 43, No. 4, December 2003, pp. 403 to 409 is known.
FIG. 12 shows a configuration of such a prior-art birefringence measuring device, which is constituted by a light source 1, an expander 2, a polarizer 3, a ¼ wavelength plate 4 for circular polarized light, a sample stand 5 on which an object to be measured is placed, three beam splitters 6A to 6C for separation to four light fluxes, ¼ wavelength plates 7-1 to 7-4 installed for each of optical paths of the separated four light fluxes, analyzers 8-1 to 8-4, two-dimensional light intensity distribution measurement devices 9-1 to 9-4 such as a CCD camera, and a mirror 10 for directing the separated light fluxes to desired directions, as necessary. Actually, the two-dimensional intensity distribution measuring devices 9-1 to 9-4 are made into a single device so as to be configured to guide light to a two-dimensional intensity distribution measuring device 9 installed at a single location by using the mirror 10 and a pyramid mirror.
However, such a prior-art birefringence measuring device requires a light amount in four polarization directions for measuring birefringence characteristics of an object to be measured and also requires a ¼ wavelength plate on each of optical paths of the four polarization directions all the time, which is indispensable and raises a device cost, and an increase in the number of components leads to a problem of a lot of labor in an adjustment work.
On the other hand, the phase shift method is employed for measurement of wavefront aberration of a lens, and a 3-phase method is known in “Optical Shop Testing” by D. Malacara, Wiley Interscience, 1992. This publicly known 3-phase method does not use polarization. This method is an algorithm in which a single sine wave created by interference is observed by changing a condition temporally or spatially (using a spatial arrangement of a sensor, for example) so that as a result, the phase is shifted and a single unknown sine wave is acquired from the observed waveforms. Thus, it is not a proposal of an algorithm for acquiring a single polarization state converted from the circular polarization through birefringence of a sample using three polarization directions.